The present invention relates to a method of coating a wooden substrate. More particularly, the present invention relates to a method of coating a wooden substrate with an energy curable polymer. Even more particularly, the present invention relates to a method of coating a wooden substrate with a structural polymer having a substantially 100% solids content which is ultraviolet light curable.
Wood is used as a durable and inexpensive building material in numerous items including doors in houses and trucks, decks, ramps, buildings, roofs, etc. An even less expensive alternative to solid wood boards is plywood or fiberboard. Wood by its very nature is a porous material and tends to decay when exposed to the elements such as rain, snow, ice, wind, sun, etc. To preserve wood, numerous types of treatments and coatings have been developed. For example, wood can be treated with chemicals such as a copper solution or alternatively be painted. When wood is painted, the surface is typically primed with a solvent-based primer to prepare the porous surface for a finishing coat of paint. The primer is necessary due to the surface characteristics of the wood. The primer and paint work to seal the wood surface from exposure to the elements and additionally work to fill various fissures and crevices in the surface. Typical paints contain about 20% solids and 80% solvent. When paint is applied to a wooden surface, the solvent evaporates away to leave the solid behind. Solids typically are the paint pigment and a polymer material such as latex. Problems with paints result in adhesion and shrinkage. Because the typical paint contains only about 20% solids, the paint tends to shrink upon application and drying. A porous surface such as wood is especially difficult to seal. Thus, numerous coats of paint along with the associated prolonged drying times are required to effectively seal and fill the porous surface. The multiple step processes associated with multiple coats of paints are undesirable, especially in a manufacturing setting, and are therefore to be avoided were possible.
Plywood or fiberboard offers additional problems over solid wood products. Plywood is typically made of pressed wood particles held together with glue or adhesives. Solvents used in the production of plywood often linger in the pressed wood long after the plywood is manufactured. These solvents tend to leach from the plywood and can create bubbling and blistering in a painted surface. For example, plywood or fiberboard is often used in the manufacture of truck doors that roll up when opened. The doors are formed with a plurality of wood slats individually hinged together and roll upwards upon a track connected to a door frame. It was discovered early on that fiberboard used to make these slats could not be economically painted due to the porosity of the surface and incompatibility of paint with glues and solvents present in the plywood. The roll-up truck door industry responded to the problem by covering the plywood slats with a veneer or cardstock paper covering. The covered slates or medium density overlay (MDO) could then be more effectively painted. It has been discovered that the present day technique of covering the plywood or fiberboard slats with a paper covering glued to the surface and then painting the surface does not adequately seal the door from exposure to the elements. It has also been discovered that the use of a veneer or paper covering of the slates adds a needless step in the production process of manufacturing overhead roll-up truck doors.
The present invention overcomes the above mentioned problems and others. As noted hereinabove, the present invention provides a method of coating a substrate with an ultraviolet (UV) light curable polymer or polymeric coating. The present method is particularly adapted for coating a "wooden" substrate and, in particular, the panels or slats comprising a vehicular wooden roll-up overhead door. The present invention enables the coating of such a wooden substrate with a structural polymer overlay having a substantially 100% solids content, in an ultraviolet light curable formulation, to create a waterproof, weatherproof, pore-bridging, grainfilling, surface strengthening member. The coating may be used as a single coat or as a layer suitable for receiving topcoats without primers. As noted, the present invention is particularly adapted for manufacturing vehicular overhead roll-up wooden doors.